Numerous conventional methods and devices are available for detecting particles such as, for example, soot or dust particles.
The present invention is described below, without limitation to additional specific embodiments and specific applications, in particular, with reference to sensors for detecting particles, in particular, soot particles in an exhaust gas flow of an internal combustion engine.
Conventionally, a concentration of particles is measured, for example, soot particles or dust particles in an exhaust gas, with the aid of two electrodes which are situated on a ceramic. This may take place, for example, by measuring the electrical resistance of the ceramic material separating the two electrodes. More precisely, the electrical current is measured, which flows between the electrodes when a voltage is applied to them. The soot particles accumulate between the electrodes due to electrostatic forces and in time form electrically conductive bridges between the electrodes. The more of these bridges that are present, the more the measured current increases. Thus, an increasing short circuit of the electrodes is formed.
Such sensors are employed, for example, in an exhaust gas system of an internal combustion engine such as, for example, a diesel-type combustion engine. These sensors are normally located downstream from the outlet valve or of the soot particle filter.
Despite the numerous advantages of the conventional devices for detecting particles, they still have the potential for improvement. The particle sensor is used to determine the soot mass in the exhaust gas tract for monitoring diesel particle filters. The sensor includes a ceramic sensor element, which is surrounded by a protective tube. The ceramic sensor element includes an electrode system, which is used to measure the soot on the basis of its electrical conductivity. The protective tube is used for, among other things, guiding a flow of the measuring gas along the soot-sensitive surface of the sensor element. A protective tube design frequently pursued is based on a dual protective tube, i.e., an outer protective tube and an inner protective tube. An angular independence of the sensor is achieved by an annular gap as an exhaust gas inlet gap between the outer protective tube and the inner protective tube, since the inflowing exhaust gas is uniformly distributed over the space between inner and outer protective tube. The exhaust gas inlet into the inner protective tube is formed in such a way that a preferential flow in the direction of the seal packing is achieved. The exhaust gas entering into this area must then flow longitudinally along the sensor element in the direction of the outlet due to the funnel-shaped design of the inner protective tube. In this way, the protective tube creates a uniform flow over the sensor element in a longitudinal extension direction of the sensor element along the main electrode device while the angular dependence is simultaneously reduced.
The disadvantage of this uniform, laminar flow-over is that although many soot particles skim past the electrode system, the electrode system is not necessarily fully charged with particles. Only particles, which flow in layers close above the electrode surface, experience sufficiently strong attraction forces as a result of electrophoresis and thermophoresis perpendicular to the main flow direction and are thus attracted and form successive soot paths. However, this occurs only in layers close to the electrode surface. If the electrical field is strong enough, an attraction rapidly occurs and the particles no longer move far in the longitudinal extension direction of the sensor element, but land instead on the electrode system. All of the particles in higher layers—the distance between the electrode surface and the inner protective tubing amounts to a few millimeters—however, experience no forces or only very weak forces perpendicular to the direction of flight and therefore skim completely past the electrode system and leave the protective tubing again. Hence, these particles do not contribute to the measurement effect. If, on the other hand, an attraction does occur, it happens very rapidly and only the front area of the electrodes are acted upon. Hence, the available electrode surface is not fully utilized.